libvulkan: Use a stub HAL when no real Vulkan HAL is present

This stub HAL enumerates zero VkPhysicalDevices. This allows a
VkInstane to be created and queried for physical devices successfully
even on devices without a Vulkan driver. Handling this with a stub HAL
avoids the need for NULL HAL and NULL driver function pointer checks
in many places throughout the loader, which would be more error-prone.

Fixes bug: 28100673
Change-Id: I76bea975929a85eda354730d6c815567b412b160